As known in the art, a semiconductor laser or a gas laser having a wavelength range between 0.6-0.8 .mu.m is often used as a light source in an optical system such as an optical communication system, an optical measurement system, and a magneto-optical disk. An outgoing light beam emitted from the laser is partially fed back to the laser itself as a return light beam to result in wavelength fluctuation and a noise. In this connection, an optical isolator is put into practical use to cut off such return light beam resulting in a noise.
A conventional optical isolator of the type described includes a single crystal birefringent element and a Faraday rotator. For use as the Faraday rotator, proposal is made of a semimagnetic semiconductor represented by a general formula of Cd.sub.1-x Mn.sub.x Te (where 0&lt;x.ltoreq.1).
The conventional optical isolator generally comprises a combination of three or four birefringent elements and two Faraday rotators at most. As one of such optical isolators comprising a plurality of birefringent elements and a plurality of Faraday rotators, a polarization-independent optical isolator is known.
In the conventional polarization-independent optical isolator, isolator characteristics tend to deteriorate in case when the Faraday rotators per se have characteristic variation due to temperature change or characteristic fluctuation due to working precision or when fluctuation occurs in an oscillation wavelength of a laser used as a light source. In the current status of the art, any appropriate technical measure is not proposed to avoid such deterioration in the isolator characteristics.